This invention relates to a method of making elongated bubble-free glass articles, and more particularly to a method of making optical fibers, especially gain fibers that are employed in fiber amplifiers.
So-called outside processes, which include the outside vapor deposition (OVD) method and the vapor axial deposition (VAD) method, produce homogeneous and optically excellent glass preforms; moreover, these methods are cost efficient because preforms of large dimensions can be produced. Both OVD and VAD methods employ burners to react precursor materials such as halides and organometallic compounds to produce a stream of glass particles or soot.
In the OVD process the soot stream deposits on the outer peripheral surface of a mandrel and builds up radially to form a porous body. After the mandrel has been removed from the porous body, it is inserted into a consolidation furnace where it is dried and sintered. A chlorine containing gas mixture is flowed through the furnace to the heated preform to dry the porous body. Helium is then flowed through the furnace and also into the preform aperture to remove residual chlorine and maintain an open centerline in the core perform during sintering at temperatures that are usually in the range of 1440-1525.degree. C. for high silica content glass. The resultant dense glass preform can be immediately drawn into an optical fiber if it contains the proper ratio of core and cladding glass. The dense glass preform often contains no cladding glass or only a portion of the required thickness of cladding glass. Such preforms are inserted into a draw furnace where the longitudinal aperture is evacuated while the tip thereof is heated and stretched into an elongated rod. The aperture closes and forms a "centerline collapse region". The rod can be severed into core canes that are overclad with additional cladding glass to form draw blanks that are drawn into optical fiber.
In the VAD process the soot stream deposits on the end of a target rod and builds up axially to form a porous body that is similar to that produced by the OVD process except that it has no axial aperture. Therefore, the dense glass VAD preform has no centerline collapse region. The remainder of the fiber producing process is similar to that described above in that the porous body is dried, sintered to dense glass, and drawn into fiber.
Some helium to which the porous body is subjected in the consolidation furnace remains in the glass in molecular form, i.e. no bubbles can be seen. Germania-containing, bubble-free overclad draw blanks have been held in 800.degree. C. ovens to outgas any trapped helium molecules and improve fiber drawability. However, some draw blanks contain visible gas bubbles that contain helium and/or other inert gases and, in some instances, additionally contain oxygen. Fibers drawn from blanks containing bubbles break or experience diameter variations during drawing. If only short lengths of fiber are needed, fiber can be drawn between bubbles; however, initiating the draw start-up process each time the fiber breaks is costly and time-consuming. A draw blank containing bubbles that are spaced along its length can be essentially useless if long lengths of transmission line fiber are to be drawn from it.
Optical fibers doped with rare earths such as erbium are commonly used in fiber amplifiers. The cores of such gain fibers usually contain GeO.sub.2 to increase refractive index. Alumina is advantageously added to the core to secure such improvements as reducing gain ion clustering and sometimes for improving the shape of the gain spectrum. It might therefore seem to be desirable to use alumina rather than germania in amplifier fibers to achieve the desired optical characteristics including increasing core refractive index. However, alumina causes crystallization problems. Standard OVD and VAD sintering temperatures produce crystallization in blanks containing more than some maximum permissible alumina concentration, depending upon glass composition and processing conditions (see U.S. Pat. No. 5,262,365). A nucleation site for such crystallization appears to include the centerline collapse region of sintered OVD preforms. Crystallization can promote trapping of gases. Bubbles composed of helium and oxygen have been routinely observed in alumina doped sintered glass preforms.
According to the SiO.sub.2 --Al.sub.2 O.sub.3 equilibrium phase diagram, the porous soot body must be sintered above the eutectic temperature of 1587.degree. C. to avoid crystal nucleation and growth. That exceeds the operating temperature of silica based muffles. Although crystals can be melted in furnaces at temperatures above the eutectic point for mullite and cristobalite as evidenced by the Al.sub.2 O.sub.3 --SiO.sub.2 equilibrium phase diagram, the resulting glass has unacceptably high bubble density and poor drawability or is undrawable.
Essentially bubble-free draw blanks are needed for drawing long lengths of distributed fiber amplifier fiber. Only relatively short lengths of rare earth doped fibers are needed for discrete fiber amplifiers. However, the fibers must be free of defects such as bubbles composed of trapped gasses (also referred to as seeds). Even if lengths of discrete gain fiber can be drawn between bubbles, the process becomes costly when the draw blank contains too many bubbles.
Attempts have therefore been made to reduce the occurrence of bubbles and crystallization in draw blanks. Dopants such as fluorine and P.sub.2 O.sub.5 have been added to the core to reduce the risk of crystallization by the alumina. However, adding additional dopants can increase cost, and the presence of such dopants in the core is usually undesirable.
Therefore, a process is needed whereby bubble-free or low bubble content alumina containing dense glass draw blanks can be produced without introducing additional undesirable dopants into the core region.
Prior art processes typically have the alumina at a maximum at the centerline. See U.S. Pat. Nos. 4,923,279; 5,058,976; and 5,155,621 where the concentration of alumina is highest at the center of the core. In the '279 patent alumina is used to adjust the refractive index of the fiber and also inhibit loss of fluorescent dopant during processing. In the process disclosed in the '976 patent the core has regions beginning with the center having alumina and germania, followed by a subsequent region comprising erbium and alumina and then a third region comprising only germania dopant. The erbium is not at the center but is in an annular ring of a radius about half that of the core. The erbium diffuses into the central alumina layer but does not reach the center of the fiber. In the '621 patent alumina is limited to the center of the core and erbium is uniformly doped through the whole body of the core. It is reported that such an arrangement reduces spontaneous emissions. OVD produced draw blanks having this type of alumina concentration profile are especially at risk of crystallization at the centerline collapse region.